This invention relates, in general, to processing semiconductor wafers, and more particularly, to methods for doping semiconductor wafers.
Impurity materials are introduced into semiconductor wafers for a variety of reasons, including metallurgical junction formation, tailoring of active device characteristics such as current gains and breakdown voltages, and setting capacitance and resistance values. A common term applied to introducing impurity materials into semiconductor wafers is doping the semiconductor wafers. The two predominant techniques for doping a semiconductor wafer are diffusion and ion implantation. Diffusion is typically a two-step process comprising a first, or predeposition, step followed by a second or drive-in step. In the predeposition step, a semiconductor dopant is deposited on a surface of the semiconductor wafer, wherein some of the dopant diffuses into the semiconductor wafer. Since the surface dopant concentration remains substantially constant as the dopant diffuses into the semiconductor wafer, the predeposition step is referred to as a constant-source diffusion. A constant-source diffusion results in a dopant profile having a complementary error function distribution.
In the drive-in or redistribution step, the predeposited dopant is further diffused into the semiconductor wafer. During the drive-in step, the surface dopant concentration decreases as the dopant diffuses into the semiconductor wafer. Thus, the drive-in step is referred to as a limited-source diffusion. A limited-source diffusion results in a dopant profile following a Gaussian distribution. Although the two-step diffusion process is a useful doping technique, it is limited by the solid solubility limit of the dopant in the semiconductor wafer.
A second method for doping a semiconductor wafer is ion implantation. In the ion implantation method, a dopant ion is accelerated towards the semiconductor wafer and penetrates the semiconductor wafer upon impact. The depth of penetration is governed by the energy imparted to the ion during acceleration as well as the size of the ion. The depth of penetration is described by a projected range R.sub.p and a standard deviation of the projected range, .DELTA.R.sub.p. The distribution of the dopant is nearly Gaussian in shape with a maximum dopant concentration at R.sub.p. Limitations of this technique include a high cost for performing high dose implants.
Accordingly, it would be advantageous to have a method for doping a semiconductor wafer in excess of the solid solubility limit of dopants in a semiconductor wafer thereby providing a high surface dopant concentration. Further, it is desirable that the method optimize manufacturing costs while employing standard doping techniques.